1. Field of the Invention
This invention pertains to free-flying power plants of use as aerial targets, projectiles or aircraft engines More particularly, this invention pertains to a relatively low drag, completely free-flying, tubular vehicle operable to achieve supersonic combustion in hypersonic flow at atmospheric pressure.
2. Description of Related Prior Art
To propel a free-flying vehicle has generally involved producing a fuel-air mixture and combusting the fuel-air mixture to produce thrust against the vehicle. The flight speed of such a vehicle is proportional to the flow speed of the fuel and air relative to the vehicle and to the speed of combustion of the fuel-air mixture.
Prior art free-flying vehicles have included conventional ramjets, supersonic combustion ramjets, ram accelerators and external propulsion accelerators.
Ramjets comprise a cylindrical cowling around, and spaced apart from, an elongate centrebody. Flow enters a conventional ramjet between the cowling and the centrebody and does so supersonically. However, that flow is made to pass through a shock wave. Three drawbacks result from such passage: (i) combustion of fuel-air mixture takes place at subsonic conditions, (ii) there is significant time for mixing to take place between the fuel and the air and (iii) flame holders are required to stabilize the combustion zone. As a result, conventional ramjets have achieved maximum flight speeds of approximately only Mach 6.
To overcome those drawbacks, and to reach higher flight speeds, supersonic combustion ramjets were developed. In such ramjets the following occurs: (i) rapid shock heating of fuel-air mixture to a temperature greater than its ignition temperature and (ii) subsequent rapid combustion leading to a standing detonation wave downstream from where the fuel is introduced, the standing detonation wave being stabilized over the operational range of flight speeds. As a result, with such ramjets the fuel-air mixture combusts supersonically producing flight speeds up to approximately Mach 8.
Still higher flight speeds are desired, so ram accelerators have been developed. A ram accelerator is basically a ramjet-in-tube device comprising a projectile in a tube. The projectile resembles the centrebody of a ramjet and is accelerated by the release of combustion energy in a stationary tube filled with a combustible gas mixture at approximately 16 to 31 atmospheres. The tube acts like the cowling of a conventional ramjet and the energy release process moves along with the projectile. The projectile can reach speeds greater than Mach 8.
However, ram accelerator combustion requires specific relative shapes and sizes for the projectile and tube. Passage of the front portion of the projectile through the ga must not only compress the gas, without igniting it, but also result in a shock wave being reflected off of the tube. The reflected shock wave then produces a standing detonation wave, outside and behind the projectile, which propels the projectile. As well, it is not clear whether a ram accelerator would work if the reactive gases were not initially at elevated pressure. The use of the tube and of the pressurized gases in the tube therefore means that the projectile cannot be completely free-flying.
External propulsion accelerators are an improvement on ram accelerators. External propulsion accelerators are like ram accelerators in two ways. First, a projectile resembling the centrebody of a ramjet is accelerated by the release of combustion energy in a stationary tube filled with a pressurized combustible gas mixture further compressed by the passage therethrough of the front portion of the projectile. Second, the energy release process moves along with the projectile. However, unlike a ram accelerator, the tube is not used to produce a reflected shock wave and thence to produce a standing detonation wave to propel the projectile. Instead, a standing detonation wave is produced by a blunt step or notch on the outside of the middle of the projectile. The blunt step or notch ignites the gas already compressed by the forward portion of the projectile. It has therefore been theorized that if on-board fuel could be effectively introduced into combustion zones external to the projectile then the projectile could be completely free-flying and reach speeds greater than Mach 8. However, a disadvantage is that the blunt step or notch of the projectile presents a large cross sectional area to the oncoming flow, resulting in very significant levels of aerodynamic drag and hence in reduced flight speed and high specific fuel consumption.